using System;

using System.Text.Json;

public class Program

{

    public class R 

    {

        public string A { get; }

        public R(string a) => A = a;

    }

    public static void Main()

    {

        var json = @"{ ""A"": ""str"" } ";

        var r = JsonSerializer.Deserialize<R>(json);

        Console.WriteLine($"Hello World {r.A}");

    }

}